Study of the adsorption kinetics of thiol-derivatized porphyrin on the surface of gold electrode

The adsorption kinetics of thiol-derivatized porphyrins (abbreviated as H2TPPO(CH2)nSH, where n = 3, 4, 6, 9, 10 and 12) on gold electrode have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and the intention was to study the effect of alkyl chain length and terminal group on the process of forming SAMs. The time dependence on surface coverage supported that the adsorption of H2TPPO(CH2)nSH molecules typically processes with a two-step adsorption consisting of a fast initial adsorption and a following slow reorganization. The equilibrium constant (K) for the adsorption and the Gibbs free energy (ΔGads) of SAMs were evaluated based on the Frumkin isotherm and the interaction factor between adsorbate–adsorbate molecules was also discussed. The adsorption processes at the initial step could be satisfactorily described by simple Langmuir adsorption kinetics, irrespective of alkyl chain length of adsorbate molecule. Based on Langmuir kinetics, the adsorption rate constant was determined at the initial step for the formation of all H2TPPO(CH2)nSH–SAMs studied in this work. The rate constant value was found to decrease with increasing alkyl chain length and the unique structure of porphyrin moiety as a large terminal group had a great influence on the initial adsorption kinetics compared with that of n-alkanethiol on gold.